Hi Daniel
I think I point out in the book that the Genesis 6 passage is heavily abbreviated from what would, in all probability, have been a familiar tale (which is why the author could abbreviate it). That makes all modern interpretations fraught with risk.
I take my non-judgemental approach to the intermarriage from writers like Richard Middleton, who in The Liberating Image points to 6:1 as humanity “literally fulfilling the injunction of 1:28 to ‘increase’ or ‘be great’ (the verb raba) on the earth,” but suggests that is ironically contrasted in 6:5 with their simultaneous increase in wickedness.
Verse 3 could refer to either side of that contrast, and if one takes on board Middleton’s analysis of the whole protohistory, of which this is a part, then it will be related to man’s (bad) sin rather than his (good) obedience to the creation ordinance.
Neither need v3 refer to the curtailing of patriarchal age: way back in time Augustine interpreted v3 not as a reduction in age, but as a pronouncement that the Flood would occur in 120 years.
I’d say the question of incest is only part of the importance of this: in my understanding via GAE, the purpose of Adam’s calling was to spread the new covenant relationship with God through the whole race, mainly through procreation. If ch6 is about human intermarriage of “gardenites” and “outside the gardenites,” rather than angels, then what God intended was either (a) if 6:1-2 represents an evil, an incestuous new race excluding those outside the garden from the blessing or (b) if 6:1-2 represents a good, a filling of the world with Adam’s seed for mankind’s blessing (but contingently, sharing the curse of sin). In the latter case, of course, incest is neither necessary nor desirable.
As too the patriarchal ages, as you’ll read further on, it remains a matter in question. The text itself (barring one of the interpretations of 6:3) says nothing about the significance of any of its ages, but simply records them. As readers, we note that the ages are less further on, but as point out in Generations, the Table of Nations shows that Adam’s line came to include most of the ANE at a time when ages are still recorded as being impossibly high by today’s standard. yet there is no archaeological or historical evidence that life expectancy in bronze age Asia and Africa was any greater than now - in fact, it’s universally agreed to be far less. Remember that, apart from his election, Abraham’s genealogical heritage from Noah was no different from someone in Put or Libya or Gomer.
For this and other reasons, I take the patriarchal ages to be either literary device of forgotten significance, or (preferable to me) a set of transmission errors possibly occasioned by transferring ancient Mesopotamian traditions to newer numbering systems.